JPS58131551A - Sensor - Google Patents

Abstract

PURPOSE:To obtain a highly reliable sensor which is prevented from pollution due to impurities and has stable characteristics and detects oxygen concentration or humidity in atmosphere by forming an oxide film including fine cracks on a gas detecting part by flame spray. CONSTITUTION:An oxide film including fine cracks consisting of oxide with <=20mum grain size is formed on a semiconductor detecting part 4 of a measuring gas detecting part of an electric insulating base 6 made of a highly heat-conductive material such as alumina which is stable at a high temp. by flame spray of plasma such as perovskite structure LaCrO3. Thus the highly reliable sensor which is prevented from pollution due to impurities and has stable characteristics and detects oxygen concentration and humidity is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel sensor, and more particularly to a round sensor for detecting and controlling oxygen concentration or humidity in a gas.

Recently, sensors that detect oxygen 11 degrees or humidity in the atmosphere and exhaust gas from automobiles, control of air-fuel ratio in phosphorus combustion equipment such as gas and oil burners, safety devices for preventing incomplete 1φ and misfires, and for automobiles. It is desired to develop a highly reliable and inexpensive oxygen concentration detection sensor that can detect the oxygen concentration in exhaust gas and can be used for controlling the air-fuel ratio.

Conventionally, as this type of sensor, oxygen concentration cell systems using stabilized zirconia, ceria, etc. have been put into practical use, but these have drawbacks in terms of structural design and installation, as well as inability to operate stably at temperatures below 350C. The disadvantage of these is that A30. This problem was overcome by the development of sensors using perovskite crystals. However, the characteristics of this type of sensor depend strongly on the oxide composition and are greatly influenced by the presence of trace impurities.

Therefore, in order to avoid contamination during the manufacturing process of this type of sensor substrate Fi, #! ! It is made by mixing with crystalline or glassy material. In the sintering method, a block of a certain size is produced and then cut into predetermined thin pieces. However, due to poor sintering properties, peropskite oxide alone is brittle, has a slow response time, makes it difficult to obtain a durable product, and has a poor appearance. It is difficult to form microscopic cracks that increase the surface area of the metal, and the accuracy of the response speed is low. On the other hand, with the firing method.

Characteristics vary over a wide range due to the inclusion of impurities, and there are also problems with one yield and characteristic deterioration during long-term use.

In response to the above-mentioned situation, it is an object of the present invention to provide a highly reliable sensor, particularly an oxygen or humidity sensor, which has stable characteristics with less contamination by impurities. In the present invention, an oxide film containing particularly fine cracks is formed on an electrically insulating substrate. This oxide is preferably one whose electrical resistance changes by about 1 to 4 orders of magnitude depending on changes in oxygen concentration and humidity, and an oxide having a perovskite structure is preferable.

In the present invention, the electrically insulating substrate is preferably a highly thermally conductive material that is stable at high temperatures, such as alumina or silicon carbide. The thickness of the board should be about 0.5W (0.3m)
It is more preferable to have less heat conductivity, which has lower thermal conductivity.
Alternatively, if it is too thick, it is likely to crack due to thermal shocks such as rapid heating to high temperatures or repeated heating and cooling.1 Durability will be poor.

As a perovskite structure oxide, a p-type oxide semiconductor is generally applied to the part that detects, senses, and responds to the fuel condition, and the main examples are LACrOJLm, Cub,
, CeTi0m+PrTi0. , NdTiQ,.

L JIN 'Om+ L a'l” OH@ La,
-x 8 'wc0OB (0,1≦X≦0.5)
#: La1-x Sr*VO1 (0≦X≦0.5)
These particles are prepared to the desired composition by conventional manufacturing methods and pulverized. The size of the particles is particularly important. The reason why a size of about 1 to 5 βm is preferable is that in perovskite structure oxide-based senna.

This is because the response speed and sensitivity depended on the thickness of the film and the state of fine cracks. On the other hand, as for the manufacturing method, as a result of examining various methods, it became clear that it is desirable to use powder in a certain particle size range and spray it by plasma spraying to form a large number of fine racks on the surface of the coating. In other words, in the thickness range of several microns to several tens of microns, the more thin microcracks there are, the higher the response speed and sensitivity will be.In the particle size range of 10 to 44 μm, which is a typical thermal spray particle size, uniform microcracks will exhibit stable characteristics. This is because it is difficult to form a thin film. Thermal spraying methods include a flame method using oxygen-acetylene and a plasma method, and the plasma method is preferable.

As a stabilizing gas for plasma spraying, a mixed gas of argon, nitrogen, hydrogen, etc. is generally used, but in the present invention, it is desirable that the gas contains oxygen. This is because the composite oxide may be reduced by high-temperature plasma during thermal spraying, resulting in a large change in properties.

Coatings with stable properties are formed by thermal spraying in an oxygen-containing atmosphere. At this time, since the film thickness affects the characteristics as described above, a crack of several micrometers or less is selected as thin as possible within the range of approximately 1 to 500 μm.

During thermal spraying, the electrically insulating substrate described above was heated to about 500C.
Ability to preheat and maintain high temperature even during thermal spraying! preferable. By doing so, it is possible to create a large number of cracks on the surface of the coating, thereby allowing close contact between the substrate and the oxide coating, thereby providing durability.

It is also possible to maintain the substrate temperature during thermal spraying lower than the above and form an adhesive oxide film that is less likely to peel off or wear out during use. it is.

The surface of the electrically insulating substrate is roughened and an oxide film is provided.
This is done by covering the surface with fine cracks or porous layers. The substrate may be provided with a desired roughness during manufacture, or a rough coating may be formed on a smooth substrate by a method such as thermal spraying. For example, the substrate is thinly coated with alumina, alumina-magnesia spinel, etc. by a thermal spraying method, or fine cracks are further generated in the coating layer. If an oxide film whose electrical resistance changes significantly in a gas atmosphere is formed on such a substrate, a highly durable sensor that will not peel off from the substrate due to thermal cycles can be obtained. Furthermore, K is used to increase response speed and improve durability.

An oxide film having higher electrical resistance and lower reactivity than alumina-based, zirconia-based, etc. is provided on the surface of the oxide film, and fine cracks are formed in the oxide film. The surface oxide layer acts as a protector or catalyst for the inner oxide film from wear and tear during use, and also functions to stabilize resistance changes. Note that the same effect can also be obtained by forming a film by mixing fine oxide powder with this alumina or the like.

Embodiment FIG. 1 shows an example of an oxygen sensor according to the present invention, in which platinum printed electrodes 4° 4' are welded to the surface of an alumina substrate 6. between this electrode.

This is a 1000x microscopic photograph of a perovskite-structured oxide powder (particle size 1 to 5 μm) containing LaNtQ, which is sprayed with an oxygen-containing plasma stream and has a thickness of about 5 μm. . A crack of 0.2 m or less is formed on the surface. This product was placed in a propane combustion flame and the change in electrical resistance was measured using the air/fuel ratio as a parameter.The results were as shown in Figure 3.

The resistance value changes by about three orders of magnitude around the position where the air/fuel ratio is 1.05. That is, the resistance value decreases exponentially as the air-fuel ratio increases in the temperature range of 400 to 900C. This phenomenon was reversible and repeated with extremely high reproducibility. Also, the temperature inside the flame is 400-900C.
The results of measuring the change in resistance value for each air/fuel ratio over a wide range are shown in FIG. Sara K.

LSI-x 8r, COO, or Lllll-x S
I'm sure a similar trend can be obtained even when it is composed of '*VO1.

Next, as shown in FIG. 1, KLaN film 0.0. The following thermal spraying conditions and configuration were used to form the coating.

(a) Preheat the substrate to 100C and spray the oxide.

(b) Preheat the substrate to 700C and spray.

(C) Preheat the substrate to 100C and add alumina (particle size 5~
37 μm) to a thickness of 10 μm or less, and then the oxide was sprayed.

LaNi0. formed in (a), (b), (c).
The thickness of the coating was 115 μm between 5 and 15 μm.

(d) After forming an oxide film with the same specifications as in (C), coat with alumina (same particle size) to a thickness in the range of 5 to 20 μm.

(e) Preheat the substrate to 200 CK and spray a mixture of LaNiOs and alumina (weight ratio 50:50).

Thickness 20. Forms a film of am.

(f) After forming the composite oxide in (a) or (b), a 5-20 μm thick AJ? tOs, coated with MgO spinel oxide.

These manufactured test pieces were subjected to a repeated test of heating to 900C for 5 seconds with a propane flame, rapidly cooling to room temperature, and holding for 5 seconds 100,000 times. As a result, the surface layer of sample (Jl) was consumed relatively quickly and became unusable. In terms of characteristics after 100,000 cycles, (d) is the best, (el), (
C), (in order of season).

Figure 5 shows that no stable oxide film is provided on the composite oxide film (m
, (be, (C) is a graph showing the change in resistance value depending on the air/fuel ratio.0 If there is no stable oxide film on the surface, when used for a long time, the change in resistance value will oscillate due to external factors. This change will be This is prevented by a stabilizing membrane.

An example of application of the present invention will be shown below.

FIG. 6 is a cross-sectional configuration diagram showing the mounting structure of the oxygen sensor when it is mounted in a place with strong vibrations such as an automobile.

Sensitive element body is 8U8306 metal nut body 5 and hole 8
U8306 Metal cover 2. Metal (brass) connector 1
2. In order to connect and fix the silicon cable electric wire 8, heat-resistant metal bottle 3.3', heat-resistant cement filling 101 and caulking 1
1 is provided. A part of the cable wire is led out through a three-hole alumina outer tube 7 for insulation and heat resistance, and one tip 4 is electrically connected to the other end of the platinum electrode of the sensing element. . Metal connector 12 and 5U8306
It is mechanically connected to the metal nut body 5 by a screw. Such a jig-equipped frame sensor has high temperature and mechanical durability. When installed as shown in Fig. 8, it becomes possible to apply it to an internal combustion engine.

In the case of an automotive senna, the operation of the idle link in combination with a heater in a low temperature range can be performed at a constant temperature by heating with the heater.

The sensitive element body and sensitive sensor 2 of the present invention can also be applied to a combustion safety device as shown in FIG. In this application example, the signal from the element body or the sensor is input to the control circuit 19, and the control circuit 19 controls the air blower 20 or the fuel supply device 21 so that the combustion state is always in complete combustion. Even if the control is performed using the air blower 20 and the fuel supply machine 21,
This is a device that stops combustion if the complete combustion level is not reached or if there is a misfire. As shown in FIG. 8, an example of application to an internal combustion engine will be described. A sensitive sensor 15 is attached to the exhaust manifold, and a silicon cable wire 32 is held and fixed to the main body pace or vehicle body 26 with a cable holding fitting 34. Connector 33 connects to a control circuit or control device.

22 is a combustion chamber, 23 is a spark plug, 24 is an air pump, 25 is a fuel injection pump, 30 is a mixing chamber, 27 is an exhaust manifold, 28 is a catalyst muffler, and 29 is an exhaust port. As an application example of the control electric circuit, as shown in Fig. 9,
E is the power supply, R1#R.

is a fixed resistor, ■ is an integrated circuit or Mico 7, L, *L,
is the load. Here, RP is the electrical resistance value of the P-dispersed oxide semiconductor film, which changes depending on the combustion state and the oxygen concentration, and R- is the electrical resistance value of the neutral oxide film.

In the figure, Lm-Lm operates upon receiving a function signal from an IC programmed in response to changes in RP and R-, and shows an example of its application.

FIG. 10 shows an example in which the sensor manufactured in the above (f) is applied to a temperature sensor. Electrical resistance changes rapidly in a linear manner when relative 1 degree is less than 50%.

It can be seen that it is effective as a humidity sensor.

[Brief explanation of drawings]

FIG. 1 is a plan view showing an example of an oxygen sensor, FIG. 2 is a microphotograph of the surface of an oxide layer, and FIG. 3 is a diagram showing the relationship between the air/fuel ratio and the resistance value of the oxygen sensor. Figure 4 is a diagram showing the relationship between ambient temperature and sensor resistance at various air/fuel ratios, Figure 5 is a diagram showing the relationship between sensor resistance and air/fuel ratio, and Figure 6 is a diagram showing the relationship between sensor resistance and air/fuel ratio. 7 is a block diagram showing an example of a combustion safety device, FIG. 8 is a block diagram showing an example applied to an internal combustion engine, and FIG. is a circuit diagram showing an example of a control electric circuit, and Figure 1O is a diagram showing the relationship between sensor resistance value and relative humidity. 1.1'... Semiconductor sensitive part % 494'... Platinum electrode. 6... Alumina substrate, 9... Lead wire, 10...
Heat resistant separator 1)] (恍) (1)) Otsu 4' Figure 5 Sky @ Carpet 6 Mouth 7 Figure yIJ8 Figure 71 Sword Figure 10 Relative humidity ('/,) = 271-

Claims (1)

[Scope of Claims] 1. A sensor characterized in that a sprayed oxide coating is formed on a portion of the surface of an electrically insulating substrate that detects a gas of a substance to be measured. 2. The sensor according to claim 1, wherein the perovskite structure oxide powder having a particle size of 20 μm or less is formed by plasma spraying to form a fine rack. 3. The sensor according to claim 1 or 2, wherein the thermal spraying is performed by maintaining the electrically insulating substrate at a temperature of 500C or higher. 4. The sensor according to claims 1 to 3, wherein the insulating substrate has a porous surface layer. 5. The cell according to claim 1, wherein the perovskite-structured oxide layer contains another stable ceramic.
2. The sensor according to claim 1, wherein the surface of the 0, S, oxide film is covered with another ceramic film.